In recent years, worldwide attention has been focused on hydrogen as a clean energy source and as an energy source that enables energy diversification to be achieved. In particular, development of fuel-cell vehicles that use high-pressure hydrogen as a fuel source has been strongly anticipated, and studies on the development of fuel-cell vehicles have been carried out all over the world. In some of the studies, a test for practical use has already been conducted.
Fuel-cell vehicles run on hydrogen contained in a tank mounted thereon instead of on gasoline. Thus, in order to spread the use of fuel-cell vehicles, hydrogen-filling stations, at which refueling is performed instead of gas stations, are required. At a hydrogen-filling station, a hydrogen fuel tank mounted on a vehicle is filled with hydrogen supplied from a hydrogen container, in which hydrogen is stored at a high pressure. While the maximum filling pressure of a vehicle-mounted hydrogen tank is currently 35 MPa, it is desired to increase the maximum filling pressure to 70 MPa in order to increase the driving ranges of fuel-cell vehicles to a level comparable to the driving ranges of gasoline vehicles. Thus, it is required to store and supply hydrogen with safety under such a high-pressure hydrogen atmosphere. Accordingly, the pressure in a hydrogen container used in a hydrogen-filling station, that is, a high-pressure hydrogen storage tank, is currently required to be 40 MPa. If the maximum filling pressure is increased to 70 MPa, the pressure in the high-pressure hydrogen storage tank used in a hydrogen-filling station would be required to be 80 MPa. In other words, in such a case, the high-pressure hydrogen storage tank used in a hydrogen-filling station would be subjected to an 80-MPa atmosphere. It is also desired that steel materials used for producing equipment or the like of hydrogen-filling stations be capable of, for example, storing and supplying hydrogen with safety even under a high-pressure hydrogen atmosphere of 80 MPa.
However, it is known that intrusion of hydrogen into a low-alloy steel causes embrittlement. In the case where the hydrogen pressure is about 15 MPa or less, low-alloy steel plate having a sufficiently large thickness can be used. However, a hydrogen pressure exceeding about 15 MPa increases the risk of hydrogen embrittlement fracture that may occur during service. Therefore, low-alloy steels are not used and, for example, austenitic stainless steels such as SUS316L steel, which are less likely to cause hydrogen embrittlement than low-alloy steels, are used instead.
Since steel materials such as SUS316L steel are expensive and have low strengths, a high-pressure hydrogen storage tank that is designed so as to withstand a hydrogen pressure of 80 MPa needs to have a considerably large thickness. Furthermore, the price of such a high-pressure hydrogen storage tank becomes considerably high. Thus, development of a high-pressure hydrogen storage tank for hydrogen-filling stations which is capable of withstanding a pressure of 80 MPa at a lower cost has been anticipated.
In order to address the above-described issues, several techniques for using low-alloy steel materials for producing a high-pressure hydrogen storage tank have been studied. Patent Literature 1 proposes a steel for high-pressure hydrogen embrittlement resistance in which nondiffusible hydrogen is produced by using a MnS-based or Ca-based inclusion or VC as a hydrogen-trapping site in the steel in order to reduce the risk of embrittlement caused by diffusible hydrogen. Patent Literature 2 and Patent Literature 3 propose a low-alloy high-strength steel having high resistance to high-pressure hydrogen atmosphere embrittlement. The tensile strength of the low-alloy high-strength steel material is controlled within a considerably narrow range of 900 to 950 MPa by performing a tempering treatment at a relatively high temperature during thermal refining of a Cr—Mo steel. Patent Literature 4 proposes a low-alloy steel material for high-pressure hydrogen embrittlement resistance in which a V—Mo-based carbide and increase of tempering temperature are used in order to enhance resistance to high-pressure hydrogen atmosphere embrittlement. Patent Literature 5 proposes a steel material for high-pressure hydrogen storage container which has high resistance to hydrogen. Large amounts of Mo and V are added to the steel material and, during production of steel plate, stress-relief annealing is performed subsequent to a normalizing treatment for long hours to cause a large amount of (Mo,V)C to precipitate. Patent Literature 6 proposes a technique in which the amount of hydrogen intrusion is reduced by reducing the sizes of cementite particles and thereby the toughness of the base material is increased in order to reduce the risk of hydrogen embrittlement. Patent Literature 7 proposes a technique in which formation of coarse cementite particles and island-like martensite (i.e., martensite-austenite constituent (MA)) is inhibited and thereby occurrences of hydrogen intrusion and ductility deterioration are limited in order to reduce the risk of hydrogen embrittlement. The fatigue crack propagation characteristic of ordinary low-alloy steels is described in, for example, Non Patent Literature 1 and Non Patent Literature 2.